Zero capacitance random access memory (ZRAM) is a type of DRAM memory based on the floating body effect of silicon on insulator (SOI) process technology. In ZRAM, the floating body effect allows the memory cell to be built without adding a separate capacitor, as the floating body effect takes the place of the conventional capacitor.
A write operation to a ZRAM involves applying a combination of a bias to the drain and a high bias to the gate. To write a one, a medium bias is applied to the drain and gate. Impact ionization generated by the current flow will build up carriers in the body. To write a zero, a negative bias is put on the gate and no voltage is applied to the drain, which flushes the body of any existing carriers without generating any new carriers.
A ZRAM cell is read by applying a high bias on the drain and a zero voltage on the gate measuring the current. This uses the ZRAM device like a bipolar. When the ZRAM is storing a zero, the body has little or no carriers and the bipolar will not activate. When the ZRAM is storing a one, the body voltage is high and the bipolar will turn on and flow high current while at the same time refreshing itself.
A ZRAM cell may have a very high drain bias that is required to operate the cell, which presents reliability problems and also affects the read/write voltage margin. For example, typical drain bias for a ZRAM may be 3V for a read operation and 2.5V for a write operation, which provides only a small 0.5V margin between the read and write operations.